Queensland's director of public prosecutions must have found a key piece of evidence overlooked by three of the state's most senior judges in order to prove Gerard Baden-Clay intended to kill his wife Allison, a retired Supreme Court judge said on Wednesday.

Gerard Baden-Clay at the funeral of his wife Allison, before he was charged with her murder.

The Honourable George Fryberg, QC, said while the grounds of a High Court appeal over the downgrading of the former Brisbane real estate agent's murder conviction were yet to be revealed, he considered it highly unlikely DPP Michael Byrne's application would be granted.

Refusal of the application would mean the controversial manslaughter ruling made in the Queensland Court of Appeal earlier this month would remain.

Allison Baden-Clay's death was ruled manslaughter, to widespread public dismay. Photo: Supplied

"The Court of Appeal judges would have read the trial transcript cover to cover, that's what they do in a circumstantial case, but if you accept the accuracy of what the Court of Appeal said about the state of evidence it is hard to see how leave to appeal would be granted," Mr Fryberg said.

"The Court of Appeal essentially said that the only evidence available in a circumstantial case to permit an inference of intent to kill was the evidence of lies told by the accused to police and in the witness box.

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"They said that by itself is not enough, I imagine what the DPP has been doing is combing through the transcript and looking for other evidence that might help support that there was an intent to kill."

Gerard Baden-Clay was convicted of the April 2012 murder of his wife Allison following a high-profile five-week trial in the Supreme Court in 2014.

A year and a half later, however, the Court of Appeal, led by the most senior member of the Queensland judiciary, Chief Justice Catherine Holmes, sensationally ruled there was insufficient evidence to convict the former Brisbane real estate agent on the charge of murder, but sufficient evidence to prove he had killed her and disposed of her body.

They set aside the murder conviction and instead ruled Allison's death was manslaughter.

Thousands attended a Brisbane rally to protest the downgrading of the murder conviction. Photo: Scott Beveridge

It prompted widespread public outrage that culminated in a public protest rally in Brisbane attended by thousands.

On Wednesday, Attorney-General Yvette D'Ath announced Mr Byrne had decided to seek special leave in the High Court to appeal the decision and have the murder conviction reinstated.

Mr Fryberg said it was extremely rare for state prosecutors to seek leave to appeal Court of Appeal decisions in the High Court of Australia, saying the number of cases granted leave to appeal each year could be "counted on one hand".

However, he said despite the enormity of emotion attached to the case and the massive public backlash to the ruling, Mr Byrne's decision would not have been swayed by public opinion.

He said the appeal must be based on evidence.

"The DPP is established under legislation which gives him a fairly high level of independence," he said.

"He has spent some time going through the transcript and looking for arguments to appeal.

"If he can find some, the question should arise, 'should I seek leave?'

"The public interest certainly warrants it if the grounds are there."

Though admitting he had not read the trial transcript, Mr Fryberg said there was "no obvious point of law" on which he could see an appeal being granted.

"The only argument I thought even approached grounds for appeal was Rosie Batty with her argument saying there was evidence of domestic violence because there was evidence of emotional abuse," he said.

"But, even saying that, she has redefined domestic violence."

Ms D'Ath said in a statement on Wednesday that Mr Byrne intended to file the application with the High Court registry when it reopened on January 4.

"The process then provides the DPP with 28 days to lodge an outline of argument with the High Court," she said.

"The defence will then have a further 21 days to do the same."

Mr Fryberg said the application would likely be heard within the first couple of months of the year.